Electrically-actuated variable transmission film having very low haze and a visible grid in a clear state

ABSTRACT

A light attenuator that provides transparent light states and absorbing dark states for use in selectively controlling light, especially for smart glass applications. The light attenuator includes abutting areas of attenuation and transparency that form a repeat pattern or a quasi-repeat pattern. The attenuating areas are visible when the light attenuator is in the light state, but the repeat pattern is sufficiently large that a viewer looks through the attenuator and sees no haze.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/034,998, filed Sep. 28, 2020, which claims priority to Great BritainPatent Application No. 1914105.0, filed Sep. 30, 2019 and to GreatBritain Patent Application No. 1914933.5, filed Oct. 16, 2019. Allreferences, patents, and patent applications disclosed herein areincorporated by reference in their entireties.

The present invention relates to an electrophoretic device having aconstruction that provides transparent light states for use inselectively controlling light, especially for smart glass applications.

BACKGROUND OF THE INVENTION

There is a need for an electrically switchable, electrophoretic devicethat in one or more light states is transparent to visible light andprovides glass-like quality and in other light states stronglyattenuates light. Glass-like quality includes providing very goodclarity and light transmission, very low haze, minimal perception of huein the transparent state, and minimal diffraction. In the prior art, theavailable electrophoretic solutions have limitations on theirfunctionality in some cases and inherent technological obstacles inothers.

In the applicant's EP2976676 the size of apertures (transparent areas)and obstructions (light blocking areas) have their maximum size andpitch determined by the resolution of a typical viewer's eye such thatapertures and obstructions are sufficiently small that their geometricform in a face view is not apparent. In examples in the document of itstransparent state its black charged particles are concentrated andsurround discrete transparent apertures, the maximum angle subtended byan aperture to a viewer at a required viewing distance is one arcminute(corresponding to 290 microns at a viewing distance of 1 meter) andpreferably 0.6 arcminutes (corresponding to 174.5 microns at 1 meter).The subtended angle of the aperture pitch (i.e. aperture andconcentrated charged particle area) can be double these limits, but onlyto the extent that the geometric forms are not apparent on a face view.

Similarly, in the applicant's EP3281055 it is stated that the device(including smart windows) has solid polymer structures embedded in itsviewing area and the structures are on the scale of microstructure andare invisible to the eye. An example is given of a smart glass devicewith a cavity pitch of 250 microns. If viewing only from a long distancethe document allows the cavities to be up to 1,000 microns in theory,however, the document constrains the microstructures to be not visiblein use. The limiting constraint is similar to the earlier EP2976676 andis stated as the maximum angle subtended by a micro-fastener portion toa viewer at a required viewing distance is one arc minute and preferably0.6 arc minutes.

In U.S. Pat. No. 8,183,757 it is stated that when the colorant particlesare collected in the reservoir regions, the colorant particles may tintthe visible areas. The tint caused by the colorant particles may preventa neutral white or clear optical state for the displays. Devices used anopaque layer on the second electrode within each recess region. Theblack opaque layer in the recesses (or reservoirs) masks the colouredcollected particles. In related patent U.S. Pat. No. 8,384,659 ahexagonal reservoir is shown in FIG. 2E and its radius is 67.5 microns.

In the transparent light states of prior art devices there is aperceivable tint corresponding to the colour of the charged particles inthe electrophoretic ink. A viewer's perception of tint, including blacktint, is one of a uniform tinting due to the micron scale, discretedistribution, and dense distribution of apertures or obstructionsanalogous to halftone print on paper or colour displays. The latter'spixel density is sufficiently high to ensure that individual pixelscannot be resolved even when viewed up close and that the light fromadjacent pixels is integrated by the eye of a viewer seamlessly.

SUMMARY OF THE INVENTION

In embodiments a light attenuator (200, 203, 204) comprising a cell(300, 303, 304) having a first transparent substrate (190) and a secondtransparent substrate (190) defining respective viewing faces (150, 153,154) and with opposite major surfaces (i.e. juxtaposed parallel) havingtransparent electrodes (160) and spaced apart (by dimension 5) toprovide a volume there between, said volume containing transparentpolymer structure (100, 103, 104) and electrophoretic ink (1, 2, 3),said ink comprising charged particles (10, 11, 12) dispersed in atransparent fluid (15, 16, 17), said charged particles are responsive toan electric field applied to said electrodes to move between: a firstextreme light state in which particles are maximally spread within saidcell to lie in the path of sunlight through the cell attenuating thesunlight and a second extreme light state in which said particles aremaximally concentrated within the cell in locations (130, 133, 134)defined by said polymer structure to remove them from the path ofsunlight through the cell transmitting the sunlight and providing visualaccess, and in said second light state a viewing face of said lightattenuator has a visible pattern of attenuating areas (20, 24) abuttedon transparent areas (30, 34) defined by the presence and absencerespectively of said concentrated particles, wherein each of saidabutted areas has a dimension (50, 55, 60, 65) that is 0.3 mm or more,and the centre-to-centre distance of adjacent attenuating areas (40, 41)or the centre-to-centre distance of adjacent transparent areas (45, 46)is 0.6 mm or more.

Each of said areas in said pattern subtends an angle (80, 90) of morethan two arc minutes at a distance of 0.5M from said viewing face (150)and a pair of said abutted areas subtend more than four arc minutes(70).

In some embodiments the pattern is a repeating pattern and saidcentre-to-centre distances are the same as the pitch. The repeatingpattern is a switchable grid that is visible in said second light stateand indistinguishable in said first light state. In some embodiments theshortest distance or width (61) of said transparent areas is 75% or moreof said pitch and the shortest distance (51) of the attenuating areas is25% or less. Preferably the limits for the preceding rule are 80% and20% respectively and more preferably 85% and 15%.

The transparent area in said face view of an embodiment is 60% or moreof the total active (i.e., switchable) area, preferably 62% or more,more preferably 65% or more, and most preferably 70% or more, and saidvisible pattern comprises discontiguous and/or contiguous areas and isperceivable as a pattern of attenuating areas. The visible pattern issuperposed on said visual access. The colour of the pattern or grid isthe colour of said charged particles. Preferably, the superposed visiblepattern or grid is designed to be aesthetically acceptable (or pleasing)by selecting the design of the locations of said concentrated chargedparticles in said polymer structure.

One of said areas is either: monodisperse or has a distribution of sizesand/or shapes, and preferably the shape of said areas is selected tominimize the opportunities for moiré patterns in said visual access andincludes selecting shapes whose borders are defined by applying amodulation function to a geometric shape.

In some embodiments the centre-to-centre distance of adjacentattenuating areas and the centre-to-centre distance of adjacenttransparent areas is 0.6 mm or more.

In some embodiments the centre-to-centre distance is in order ofpreference: 0.62 mm or more, 0.65 mm or more, 0.7 mm or more, 0.8 mm ormore, 1.0 mm or more, and most preferably 1.25 mm or more. Incorrespondence to the preceding, the abutting attenuating areas andtransparent areas each have one or more dimensions that are in order ofpreference: 0.31 mm or more, 0.325 mm or more, 0.35 mm or more, 0.4 mmor more, 0.5 mm or more, and most preferably 0.625 mm or more.

The charged particles have colourant including one or more of: a dyecolorant, a pigment colourant, a strongly light scattering material, astrongly reflecting material, or a strongly absorbing material, andparticles can be any colour including black or white.

The polymer structure spaces apart said substrates and divides saidvolume into a monolayer of discrete cavities having polymer walls andfilled with said electrophoretic ink, and preferably said polymerstructure includes a sealing layer sealing the ink within the cavities.

In some embodiments a colour layer is selectively applied to saidpolymer walls so that in said viewing face the colour of the wall areamatches the colour of said charged particles.

In embodiments the locations of said concentrated charged particles areat said polymer walls, or the locations of said concentrated chargedparticles are in discrete reservoirs in said polymer structure and thelocations do not coincide with said walls, or the locations of saidconcentrated charged particles are in depressions or channels betweenprotrusions in said polymer structure and the locations may or may notcoincide with said walls.

The electrophoretic ink has two or more charged particle typesincluding: positively charged, negatively charged, differingelectrophoretic mobility, and/or different colours.

The electrophoretic ink has two charged particles types each with anelectrophoretic mobility and colour different to the other but the samecharge polarity, and in the second light state said two types segregateas they concentrate at said locations with one type masking the otherwith respect to one of said viewing faces.

Preferably light attenuators provide at least one light stateintermediate the first and second states by moving the charged particlesbetween the concentrating locations in the polymer structure and theopposite electrode to vary the degree of concentrating or spreadingrespectively.

A device including one of: a window, a mirror, a light shutter, a lightmodulator, a variable light transmittance sheet, a variable lightabsorptance sheet, a variable light reflectance sheet, anelectrophoretic sun visor for a vehicle, or a see-through display,incorporating the light attenuator.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 a shows embodiment 200 in a first light state. The drawing showsa view of its face.

FIG. 1 b shows embodiment 200 in a second light state and isthree-dimensional view of its face.

FIG. 1 c shows polymer structure 100 and is a three-dimensional view ofits face.

FIG. 2 a shows embodiment 200 in a second light state and is a faceview.

FIG. 2 b shows embodiment 200 in a second light state and is a faceview.

FIG. 3 shows embodiment 203 in a second light state and is a crosssectional view.

FIG. 4 shows embodiment 204 in a second light state and is a face view.

DETAILED DESCRIPTION

Embodiments achieve improved optical quality in its electrophoreticlight attenuators by making its particles visible as a pattern, orgrid-like structure, or array, in its transparent light state. In thisstate the coloured charged particles are concentrated in defined areasby a transparent polymer structure so that areas in between transmitlight. In the prior art neither the concentrated particle areas nor thetransparent areas were resolvable, but, in embodiments both areas can beresolved as distinct by eye from at least 1M away. Embodiments minimizethe integration of both areas by a viewer so that the scene viewedthrough the attenuator is significantly less tainted with the colour of,or haze from, the charged particles. A viewer's perception of the faceof an embodiment is of clear glass with a coloured grid (or array)structure. The latter can be selected to be aesthetically pleasing.Furthermore, by selecting the scale of the pattern or grid-likestructure (or array) to be visible, a light attenuator made with whiteparticles does not appear to be hazy, rather, it appears as a foregroundwhite grid superposed on the background scene. In addition, diffractionof light is greatly reduced by using a visible scale for the design ofthe transparent light state.

Embodiments are described with reference to the drawings. In FIGS. 1 ato 1 c and FIGS. 2 a and 2 b different features of the same lightattenuator (200) are shown in face views. FIG. 1 a shown the stronglylight attenuating first light state, and FIGS. 1 b, 2 a and 2 b show thetransparent second light state. FIG. 1 c shows the polymer structure ofembodiment (200). FIG. 3 shows embodiment (203) in cross section and issimilar to embodiment (200). In these embodiments the second light stateresults from the electrophoretic ink's charged particles (10, 11) beingconcentrated in channels (130, 133) between protrusions (110, 113) inthe polymer structure (100, 103) by an applied electrical field. FIG. 4shows the face view of an alternative embodiment (204) that has adifferent layout of its polymer structure (104) such that its chargedparticles (12) move across the polymer surface (117) to concentrate indiscrete reservoirs or pits (134) in the second light state.

In the figures the light attenuators (200, 203, 204) of embodimentscomprise an electrophoretic cell (300, 303, 304) that has twotransparent substrates (190) with each coated on one side with atransparent electrode (160). The electrodes' major surfaces face eachother and are juxtaposed parallel as shown in FIG. 3 . The oppositesurfaces of the substrates form the viewing faces (150, 153, 154) of thecells. The substrates are spaced apart and the volume between them isthe electro-optical layer of the device. The dimension indicating thespacing apart is (5) and is shown in FIG. 3 . In some embodimentsdimension (5) defines the cell gap. The electro-optical layer (orvolume) comprises transparent polymer structure (100, 103, 104) andelectrophoretic ink (1, 2, 3). The electrophoretic ink comprises chargedparticles (10, 11, 12) dispersed in an otherwise transparent fluid (15,16, 17). The charged particles can be any colour including black (11),white (12) or red (13). The particles respond to an electric fieldapplied to the electrodes to move between light states bounded by twoextremes. In the first extreme light state, shown in FIG. 1 a, particlesare maximally spread within the cell to lie in the path of sunlightthrough the cell so that the sunlight is attenuated and the viewingfaces are the colour of the particles. The first extreme light statecorresponds to a spread state or maximum attenuating or obscuring orhiding state.

In the second extreme light state (shown in FIGS. 1 b, 2 a, 2 b, 3 and4) the particles are maximally concentrated within the cell in locations(130, 133, 134) defined by the polymer structure (100, 103, 104). FIG. 1c shows a three-dimensional view of the polymer structure (100). Channel(130) forms an interconnected depression or space between discreteprotrusions (110) to confine concentrated particles (10) in the secondlight state. Concentrated particles can fill the channel up to the levelindicated by (115). This removes the particles from the path of sunlightthrough the cell so that the sunlight is transmitted without a colourhue (i.e., transmitted without encountering coloured particles) and thecell is see-through providing visual access through an openingincorporating the embodiment.

The second light state of cell (303) shown in FIG. 3 shows light ray(1200) from an outside environment passing through a protrusion (113)(shown hatched) corresponding to a transparent area and emerging fromthe cell as ray (1201) to illuminate an inside environment. Lightincident on a concentrated particle area is diffusely reflected by whiteparticles (11) in channel (133) in embodiment (203) as shown by incidentand reflected rays (1205) and (1206) respectively. In cell (300) shownin FIG. 2 a light incident on the concentrated particle area (20) isstrongly absorbed by black charged particles (10). In cell (304) shownin FIG. 4 light incident on concentrated particle area (24) is bothabsorbed and diffusely reflected in the visible spectrum by red chargedparticles (12). Regardless of the colour of charged particles, lightincident on the concentrated particle area is not significantlytransmitted and so this area is described herein as attenuating.

FIG. 1 b shows that in the second extreme light state the viewing face(150) of the light attenuator has a visible pattern of attenuating areas(20) abutting on (i.e. side-by-side with or juxtaposing) transparentareas (30) defined by the presence or absence respectively of theconcentrated particles that in turn are defined by the polymerstructure's channel (130) and protrusions (110) (the latter two areshown in FIG. 1 c ).

In FIG. 2 a these attenuating (20) and transparent (30) abutting areasare arranged so that each area has one or more dimensions that are 0.3mm or more (see (50) and (60)) and therefore visible or discernible orresolvable as parts (or elements) by eye. In addition, thecentre-to-centre distance (40) of adjacent attenuating areas or thecentre-to-centre distance (45) of adjacent transparent areas is 0.6 mmor more. The viewing face (150) has a plurality of abutting areas andcentre-to-centre distances and they form a visible pattern across itsarea.

Similarly, in FIG. 4 the attenuating (24) and transparent (34) abuttingareas are arranged so that each area has one or more dimensions that are0.3 mm or more (see (55) and (65)) and therefore visible by eye. Inaddition, the centre-to-centre distance (41) of adjacent attenuatingareas or the centre-to-centre distance (46) of adjacent transparentareas is 0.6 mm or more. The viewing face (154) has a plurality ofabutting areas and centre-to-centre distances and they form a visiblepattern across its area.

The 0.6 mm centre-to-centre dimension is sufficiently large to bevisible at 1.0M by a viewer with a visual acuity of 1.0 or higher as theviewing distance results in an angular resolution of two minutes of arc.The pattern is visible because the attenuating areas corresponding tothe concentrated particles in the second light state form resolvableparts (or features) when they have a centre-to-centre distance of 0.6 mmor more and transparent area in the space between adjacent attenuatingareas. A visual analogy can be made with the graduation pattern on asteel rule; graduations having a centre-to-centre distance of 0.5 mm(i.e. 0.5 mm divisions) are visible.

While the dimensions of each area of attenuating (20) and transparent(30) abutting areas should be at least 0.3 mm or more to be discernibleto a viewer at 1.0M, and have a centre-to-centre dimension of at least0.6 mm, it is also necessary that the dimension of the each area ofattenuating (20) and transparent (30) abutting areas should not be solarge that the pigment loading of the transparent area cannot be packedinto the attenuating area. This is because, in most configurations, thearea above the transparent area increases as the square of thedimension, while the surface area of the attenuating area, where theparticles will be packed, increases roughly linearly with the dimension.If the dimension grows too big, the particles cannot be effective packedin the attenuating area, leading to a darker clear state. Largerdimensions are also found to require higher voltages to achieve goodclearing. Experience with various sized of dimensions suggests that thearea of attenuating (20) and transparent (30) abutting areas should notexceed 3 cm. The corresponding maximum centre-to-centre dimension isabout 6 cm. Thus, each repeat of attenuating (20) and transparent (30)abutting areas should have a dimension (50, 55 and 60, 65) between 0.3mm and 3 cm, while the centre-to-centre distance (40, 41) of adjacentattenuating areas or the centre-to-centre distance (45, 46) of adjacenttransparent areas is between 0.6 mm and 6 cm.

In embodiments, in the second light state, and for a plurality ofinstances, the centre-to-centre distance of adjacent attenuating areas,or the centre-to-centre distance of adjacent transparent areas is inorder of preference: 0.62 mm to 5.8 cms, 0.65 mm to 5.5 cms, 0.7 mm to5.14 cms, 0.8 mm to 4.5 cms, 1.0 mm to 3.6 cms, and most preferably 1.25mm to 3 cms, and in correspondence to the preceding, the abuttingattenuating areas and transparent areas each have one or more dimensionsthat are in order of preference: 0.31 mm to 2.9 cms, 0.325 mm to 2.75cms, 0.35 mm to 2.57 cms, 0.4 mm to 2.25 cms, 0.5 mm to 1.8 cms, andmost preferably 0.625 mm to 1.5 cms.

In embodiments the transparent area in the face view is 60% or more ofthe total active (i.e., switchable) area, preferably 62% or more, morepreferably 65% or more, and most preferably 70% or more. The attenuatingarea is the remainder in each case. The transparent areas, theattenuating areas, and accordingly the resolvable parts and pattern inthe second light state, are defined by a device's polymer structure.

In an example shown in FIG. 2 b, a vehicle is fitted with embodimentsfor its side windows and sunroof. The charged particles (10) are notdrawn but the corresponding attenuating area (20) is shown. Occupants ofthe vehicle would typically have a viewing distance of 0.5M (or less)through these openings in the second light state. The angle subtended byan attenuating area (20) is indicated by (80) and the angle fortransparent area (30) by the number (90). An abutted attenuating area(20) and transparent area (30) is shown subtending an angle (70). At0.5M viewing distance this abutted area pair of (20)+(30) subtends over4.1 minutes of arc or 4.1 times the minimum angular resolution of aviewer with a visual acuity of 1.0 when the distance (edge-to-edge) ofthe abutted areas is 0.6 mm or more. The angle was calculated as:

$\begin{matrix}{{{Angle}{subtended}} = {{Tangent}\left( {0.6/500} \right)}} \\{= {0.0012\left( {{in}{radians}} \right)}} \\{= {4.125\left( {{in}{arc}{minutes}} \right)}}\end{matrix}$

The corresponding calculation for subtended angles (80) and (90) at theminimum 0.3 mm dimension within the visible pattern in embodiments is2.06 minutes of arc:

$\begin{matrix}{{{Angle}{subtended}} = {{Tangent}\left( {0.3/500} \right)}} \\{= {0.0006\left( {{in}{radians}} \right)}} \\{= {2.063\left( {{in}{arc}{minutes}} \right)}}\end{matrix}$

As a consequence of the subtended angles for (70), (80) and (90) being amultiple of the minimum resolution of a viewer (with acuity 1.0) thereis an obvious visible pattern when viewing embodiment (200) at 0.5M.Attenuating area (20) (comprising black concentrated particles (10)) canbe seen as a black grid (or array) with clear openings analogous with ametal mesh having comparable openings and walls. Objects viewed throughthe embodiment have a negligible perception of black hue because theviewer's eye does not integrate the black grid area with the viewthrough the transparent areas.

By contrast, the motivation of the prior art electrophoretic, lightattenuator devices that have polymer structure throughout theirelectro-optical layers is to arrange their structures so that thestructures (or associated patterns in the light states defined by thestructures) are sufficiently small that they cannot be perceived by aviewer. In the applicant's EP2976676, the size of apertures (transparentareas) and obstructions (light blocking areas) have their maximum sizeand pitch (analogous to the repeating centre-to-centre distance)determined by the resolution of a typical viewer's eye so that at aviewing distance of 0.5M, its areas subtend an angle of less than onearc minute and this equates to less than 0.145 mm to avoid a patternbeing apparent to a viewer.

In embodiments, in the second light state, the centre-to-centre distanceof attenuating areas or transparent areas (defined by the presence orabsence respectively of the concentrated particles that in turn aredefined by the polymer structure) can be random or have more than onevalue. In other embodiments the centre-to-centre distances repeatuniformly in a direction and are the same as the pitch of the repeatingpattern that is visible by eye.

FIG. 2 a shows the pitch (1040) of abutting attenuating and transparentareas in the top-to-bottom direction. It is longer than the pitch (1041)in the left-to-right direction. In embodiment (200) the shortestdistance or width (61) of the transparent areas can be 75% or more ofthe pitch (1041) and the shortest distance (51) of the attenuating areascan be 25% or less. Preferably the limits for the preceding rule are 80%and 20% respectively and more preferably 85% and 15%. FIG. 4 shows thepitch (1045) of abutting attenuating and transparent areas foralternative embodiment (204).

In embodiment (200) the transparent areas (30) are discrete and theattenuating area (20) is contiguous, see FIG. 2 a. The reverserelationship is shown by embodiment (204) in FIG. 4 : the attenuatingareas (24) are discrete and the transparent area (34) is contiguous. Inboth embodiments the centre-to-centre distance of adjacent discreteareas is easily identified (see (45) and (41) respectively). Thecorresponding centre-to-centre distance for the contiguous area relatesto the area between the discrete areas as shown by dimension (40) inFIG. 2 a and dimension (46) in FIG. 4 . In this regard embodiments canbe defined by a centre-to-centre distance for both its attenuating areasand its transparent areas.

In the second light state the visible pattern formed by the attenuatingand transparent areas in a face of embodiments is superposed on the viewthrough the face. The visible pattern is in the foreground and the viewis in the background. The eye resolves the visible pattern as a grid (orarray) and perceives it as a grid of opaque areas that are the colour ofthe particles. In embodiments this grid can be made indistinguishable onthe face when switched to the first light state. The charged particlesin the first light state spread uniformly and opposite the locationsthat receive the particles as they concentrate in the second lightstate. Preferably, the superposed visible pattern or grid (or array) isdesigned to be aesthetically acceptable (or pleasing) by selecting thedesign of the locations of the concentrated charged particles in thepolymer structure.

An embodiment's polymer structure, including the locations of theconcentrated charged particles, is formed at least in part in anembossing, moulding or replicating step. Examples of moulding techniquesare described in the applicant's EP2976676 titled “An ElectrophoreticDevice Having a Transparent Light State”. To minimize haze inembodiments the refractive index of the polymer structure (100, 103,104) is matched to the ink's suspending fluid (15, 16, 17), preferablyto within 0.005, more preferably, 0.002, and most preferably, 0.001.

The replicated polymer structure has depressions, channels, pits,recesses, or reservoirs corresponding to the attenuating areas andprotrusions, funnel-like sloping surfaces, or a raised surface inbetween corresponding with the transparent areas. The shape of bothareas in a face view as well as the centre-to-centre distance is definedby the polymer structure. Either area type (i.e., attenuating ortransparent) can be monodisperse or have a distribution of sizes and/orshapes. Examples of devices having channels and protrusions can be foundin the applicant's EP2976676; devices having reservoirs and funnel-likesloping surfaces in HP's U.S. Pat. No. 8,184,357; and, devices havingrecesses and raised surfaces in HP's U.S. Pat. No. 7,957,054. The latterrefers to a dielectric layer with recesses but the dielectric layer is apolymer structure and its concentrated particles are located at recesses(or pits, voids, or holes) in the layer in its transparent light state.In an alternative embodiment the polymer structure provides walls thatcharged particles concentrate against in the second light state. Thesedevices are referred to as dielectrophoretic and an example is shown inE Ink's US2018/0364542 A1.

In some embodiments the shape of areas is selected to minimize theopportunities for moiré patterns that would otherwise occur if anembodiment's opaque grid (i.e., strongly attenuating areas arranged inan array) is overlaid on a similar pattern in the background viewedthrough a face. To avoid or minimize moiré patterns the attenuatingareas preferably avoid a pattern of continuous parallel lines. In thisregard a honeycomb structure as shown in FIG. 2 a is preferred as it isless likely to be similar to grid structures encountered in a backgroundscene. But in more preferred embodiments a modulation function isapplied to a geometric shape. For example, the polymer structure (110)shown in FIG. 1 c in an alternative embodiment has the border of itshexagonal shaped protrusions (110) modulated with a Sine wave to havewave-like sides instead of flats. The visible pattern of thisalternative embodiment is characterized by wave-like segments instead ofstraight segments and so less likely to cause moire patterns whenviewing a scene.

In some embodiments two or more devices are stacked and to avoid moirepatterns each device has a different grid (or array) pattern. In anembodiment example, a sunvisor in a vehicle comprises a stack of twodevices to achieve very low light transmittance when both devices areoperated in their respective maximum attenuating light states. Theembodiment achieves a corresponding maximum light transmitting statewhen both devices are in their light transmitting states. To avoid moirepatterns in some embodiments both devices' attenuating areas (andtransparent areas) are precisely aligned, but, in preferred embodimentsthe shape of light attenuating areas is selected to be different betweendevices. For example, one device has a honeycomb structure for itsattenuating area and the other device has a monodisperse shape such asspherical, or one whose border is modulated by a Sine wave.

In embodiment (203) shown in FIG. 3 the polymer structure (103) spacesapart the light attenuator's substrates using integrated posts (123).The posts (123) define the cell gap and the orthogonal distance betweenthe substrates is shown as dimension (5). The cell is sealed all aroundwith a polymer seal (not shown). Post (120) on polymer structure (100)shown in FIG. 1 c has a similar function. In other embodiments thepolymer structure uses polymer walls to space apart the lightattenuator's substrates and divide the volume there between into amonolayer of discrete cavities tilled with an electrophoretic ink. Thewalls in this case define the cell gap.

In embodiments the cell gap (dimension (5) in FIG. 3 ) is from 7.5microns to 300 microns, preferably from 13.5 microns to 200 microns,more preferably from 16.5 microns to 150 microns, and most preferablyfrom 18 microns to 125 microns.

Preferably, a colour layer is selectively applied to the tops of polymerwalls and/or posts so that in a viewing face the colour of the wall areamatches the colour and light transmission of the attenuating areas inthe second light state. Preferably the polymer structure includes asealing layer or sealing mechanism that seals the fluid within eachcavity. The seal layer preferably bonds to the colour layer on thepolymer walls (or incorporates the colour layer), In some embodimentssealed cavities are independent of one another and can be described ascells, and the light attenuator as comprising a monolayer of cells.

In some embodiments the polymer structure locates the concentratedcharged particles against or by its polymer walls including in channelsadjacent its walls in the second light state. In such embodiments thereare concentrated particles on each side of a polymer wall section forthe respective cavities each side. Preferably the polymer walls have anattenuating layer and are coloured to match the particles; then theattenuating area for the concentrated particles will appear contiguouson a face of the device and the transparent areas will be discrete.Optionally, the polymer walls can be transparent, and if so arepreferably as narrow as possible and preferably the width is in therange: 15 microns to 75 microns. In the latter case the attenuatingareas in the second light state are discontiguous.

In other embodiments the polymer structure locates the concentratedcharged particles in discrete reservoirs that do not coincide with thewalls in the second light state. The attenuating areas are discrete andsurrounded by contiguous transparent area. Preferably the walls are asnarrow as possible and remain transparent so that the attenuating areain the second light state appears contiguous. Alternatively, the wallsmay have a colour layer.

In more preferred embodiments the concentrated charged particles are indepressions or channels between protrusions in the polymer structure andthe locations may or may not coincide with the walls. The transparentareas are discrete and the attenuating areas are contiguous. Preferablythe walls have an attenuating layer in embodiments where they coincidewith locations of the concentrated particles, or, are transparent wherethey do not.

Cavities can contain a single transparent area and a single attenuatingarea or a plurality of either, or a part of either. Cavities can beuniform and repeat with a pitch or have differences. Thecentre-to-centre distance between adjacent cavities can be greater than,equal to, or less than, the centre-to-centre distance of transparent orattenuating areas defined by the concentrated charged particles in thesecond light state. The polymer walls of cavities can also form avisible grid on a face of embodiments but this grid is not switchable.It will be appreciated that it is advantageous to have polymer wallsthat have an attenuating layer arranged adjacent the locations ofconcentrated particles where possible. Alternatively, it is advantageousto have transparent walls arranged predominantly in transparent areas ofthe second light state.

In some cells a colour mask (i.e., a colour layer) different to thecolour of the charged particles is selectively applied to a surface ofthe polymer structure in the locations where particles are concentratedin the second light state (i.e. the attenuating areas). The colour maskareas correspond to the attenuating areas and consequently inembodiments form a visible pattern or grid. In the viewing face on thesame side as the colour mask the colour of the locations masks thecolour of the concentrated charged particles in the second light state.An embodiment having white charged particles can avoid diffusereflection from its attenuating areas (i.e. the concentrated chargedparticles areas) in the second light state by masking these areas with ablack mask printed on the polymer structure, or on a face of thesubstrate on the same side. Alternatively the colour mask could beapplied to the opposing area on the polymer structure or the oppositesubstrate to mask from the other viewing face. Similarly, both sides canbe selectively printed to mask or minimize diffuse reflection ortransmission from the concentrated particle area in the second lightstate. The colour mask is defined in embodiments by the locations (130,133, 134) in the polymer structure (100, 103, 104) that define theconcentrated charged particles (10, 11) in the second light state andconsequently is visible by eye when viewed from the face it is adjacentto.

In embodiments the electrophoretic ink can have one, two, or more typesof charged particles including: positively charged, negatively charged,differing electrophoretic mobility, and/or different colours, or anycombination of these. The charged particles have colourant including oneor more of: a dye colorant, a pigment colourant, a strongly lightscattering material, a strongly reflecting material, or a stronglyabsorbing material. In some embodiments the electrophoretic ink has twocharged particles types, each with an electrophoretic mobility andcolour different to the other but the same charge polarity. In thesecond light state the two types segregate as they concentrate at thelocations in the polymer structure with one type masking the other withrespect to the viewing faces on the same side. This is an alternative toapplying a colour mask to the locations as described in the previousparagraph. A minority of black charged particles with higherelectrophoretic mobility can be used to mask a different colour ofcharged particle such as a majority of white particles having a lowerelectrophoretic mobility.

Preferably light attenuators provide at least one light stateintermediate the first and second states by moving the charged particlesbetween the concentrating locations in the polymer structure and theopposite electrode to vary the degree of concentrating or spreadingrespectively. A visible pattern will be apparent in intermediate lightstates once particles begin to concentrate in the locations provided. Inembodiments where the charged particles are a colour other than black(e.g., white) haze will be at a minimum in the second light state andincrease the closer an intermediate light state is to the first lightstate. In some embodiments the first light state is very strongly hazyto provide a privacy function.

1. A light attenuator comprising: a first transparent substrate; asecond transparent substrate; a first transparent electrode adjacent thefirst transparent substrate; a second transparent electrode adjacent thesecond transparent substrate, a transparent polymer structure disposedbetween the first transparent electrode and the second transparentelectrode, the transparent polymer structure defining a volume betweenthe first transparent electrode and the second transparent electrode;and an electrophoretic ink disposed in the volume, the electrophoreticink comprising charged particles dispersed in a transparent fluid,wherein the charged particles are responsive to electric fields appliedbetween the first and second transparent electrodes and with theapplication of electric fields the charged particles move between: afirst state in which the charged particles are distributed throughoutthe volume, thereby attenuating light passing through the lightattenuator, and a second state in which the charged particles areconcentrated within locations in the transparent polymer structure,thereby removing the charged particles from the path of light passingthrough the light attenuator, thereby providing visible transmissionbetween the first and second transparent substrates, wherein in thesecond state, a viewing face of the light attenuator has a dark patternof locations adjacent transparent areas, the dark pattern andtransparent areas being defined by a presence and an absence,respectively, of concentrated charged particles, the dark patterncomprising repeating units of locations of concentrated chargedparticles, and wherein each repeating unit of the dark pattern has adimension between 0.3 mm and 3 cm, and a centre-to-centre distancebetween repeating units between 0.6 mm and 6 cm. The light attenuator ofclaim 1, wherein in the second state, the transparent area in a faceview is 60% or more of a total active area in the face view.
 3. Thelight attenuator of claim 1, wherein the dark pattern is a switchablegrid that is visible in the second state and indistinguishable from thedispersed charged pigment in the first state.
 4. The light attenuator ofclaim 1, wherein the transparent polymer structure includes walls thatdivide the volume into a monolayer of discrete cavities comprising thewalls and filled with electrophoretic ink.
 5. The light attenuator ofclaim 4, further comprising a sealing layer to retain a portion of theelectrophoretic ink inside each discrete cavity.
 6. The light attenuatorof claim 4, wherein at least portion of the polymer walls are colored tomatch the color of the dispersed charged pigment.
 7. The lightattenuator of claim 6, wherein in the second state, the concentratedcharged particles are adjacent the walls of the transparent polymerstructure.
 8. The light attenuator of claim 4, wherein in the secondstate, the locations of concentrated charged particles are not adjacentthe walls of the transparent polymer structure.
 9. The light attenuatorof claim 8, wherein in the second state, the locations of concentratedcharged particles comprise depressions or channels between protrusionsin the polymer structure, wherein the protrusions do not span a distancebetween the first transparent electrode and the second transparentelectrode.
 10. The light attenuator of claim 1, wherein theelectrophoretic ink comprises two types of charged particles selectedfrom the group comprising positively-charged particles,negatively-charged particles, different-electrophoretic-mobilityparticles, and differently-colored particles.
 11. The light attenuatorof claim 1, wherein the electrophoretic ink comprises two types ofcharged particles, each type each having a different electrophoreticmobility and a different color from each other.
 12. The light attenuatorof claim 11, wherein the two types of charged particles have a samecharge polarity, and wherein in the second state, the two types ofcharged particles segregate when they concentrate at the locations withone type of charged particle masking the second type of charged particlewith respect to a viewing face of the light attenuator.
 13. The lightattenuator of claim 1, wherein the charged particles are additionallystable in a third state between the first state and the second state,thereby providing an intermediate light transmission state between thefirst and second states.
 14. A light attenuator of claim 1, incorporatedinto one of: a window, a mirror, a light shutter, a light modulator, avariable light transmittance sheet, a variable light absorbance sheet, avariable light reflectance sheet, an electrophoretic sun visor for avehicle, or a see-through display.